1. Field of the Invention
The present invention generally relates to an optical scanning device and an image forming apparatus using the optical scanning device such as a digital copier, a laser printer, and the like and more particularly to an optical scanning device and an image forming apparatus using the optical scanning device that can be applied to optical scanning barcode reading devices, on-vehicle laser radar devices, and the like.
2. Description of the Related Art
Conventional optical scanning devices employ a polygon mirror or a galvano mirror as a deflector for light beam scanning rotating at a high speed. However, in order to achieve higher resolution images and high-speed printing, the rotation of the polygon mirror or the galvano mirror must be faster. Accordingly, durability of a bearing, heat generation due to windage, and noise are problematic and high-speed scanning have a limitation.
By contrast, research on deflection devices using silicon micromachining has progressed in recent years and proposed a method for integrally forming an oscillating mirror with a twist beam axially supporting the oscillating mirror using a silicon (hereafter referred to as Si) substrate (refer to Patent Document 1 and particularly to Patent Document 2). According to this method, it is possible to provide a small-sized deflection device with a small mirror surface. Also, this method has a merit in that low noise and low power consumption are possible in spite of high-speed operations since reciprocating oscillation is performed using resonance. In addition, this method has another merit in that a housing for storing the optical scanning device can be made of thin walls and even when a low-cost resin molding material with a low compound ratio of glass fiber is used as a housing material, such a material is not likely to cause a negative effect on image quality because of low oscillation and little heat generation of the deflection device. Further, other examples have also been proposed in which an oscillating mirror is disposed instead of a polygon mirror (refer to Patent Document 3 and Patent Document 4, for example).
In the following, general characteristics of an oscillating mirror are considered. FIG. 17 is a diagram showing a rectangular plate-like oscillating mirror generally assumed. Twist beams protrude from a center of faces on both ends of the oscillating mirror in a longitudinal direction and a center of the twist beams is a rotation axis. When a size of the oscillating mirror is described based on a width d in parallel with the rotation axis, a width 2r orthogonal to the rotation axis, and a thickness t, and a size of the twist beam is described based on a length h and a width a, moment of inertia I=(4ρrdt/3)·r2 spring constant K=(G/2h)·{at(a2+t2)/12}, where density of Si is ρ and material constant is G. Resonance frequency f0=(½π)·√(K/I)=(½π)/√{Gat(a2+t2)/24LI}. In this case, a length L of the twist beam and a swing angle θ0 are substantially in a proportional relationship, so that swing angle θ0=κ/I·f02 and κ is represented by a constant . . . (1). Resonance frequency f0 is changed by spring constant K of the twist beam and swing angle θ is also changed in accordance therewith.
Further, when density of air is r relative to a circumferential speed u and an area E(=2rd) of the oscillating mirror, viscous drag of air p=c·ηu^2·E^3 (c is a constant) works against rotation of the oscillating mirror.
On the other hand, a relationship between oscillation torque T and swing angle θ0 is represented by θ0=κ′·T/K (κ′ is a constant) . . . (2)
Thus, so as to stably maintain the swing angle θ, a current to be applied may be adjusted in order to generate rotation torque T in accordance with a change of spring constant K of the twist beam and air resistance.
As mentioned above, the spring constant of the twist beam is changed due to temperature and the resonance frequency is changed, or the viscous drag of air is changed due to atmospheric pressure, so that the change of the swing angle is problematic. In view of this, an optical scanning device has been proposed in which the swing angle is detected by detecting a scanned beam and the swing angle is stably maintained by adjusting a current to be applied to the oscillating mirror (refer to Patent Document 5, for example).
On the other hand, when the resonance frequency becomes high, the swing angle θ of the oscillating mirror becomes small. In view of this, certain method has been generally known in which an optical scanning speed is increased by increasing the number of luminous sources and simultaneously scanning plural lines (refer to Patent Document 6, for example). Further, an example has been proposed in which a semiconductor laser array having plural luminous sources and a resonant oscillating mirror are combined (refer to Patent Document 7, for example).    Patent Document 1: Japanese Patent No. 2924200    Patent Document 2: Japanese Patent No. 3011144    Patent Document 3: Japanese Patent No. 3445691    Patent Document 4: Japanese Patent No. 3543473    Patent Document 5: Japanese Laid-Open Patent Application No. 2004-279947    Patent Document 6: Japanese Laid-Open Patent Application. No. 10-301044    Patent Document 7: Japanese Laid-Open Patent Application No. 2005-24722
As mentioned above, by using the oscillating mirror as a light deflector instead of a polygon mirror, low noise and low power consumption are possible, so that it is possible to provide an optical scanning device and an image forming apparatus using the same suitable for an office environment. Moreover, when the housing of the optical scanning device is made of thin walls, for example, it is possible to achieve reduction in weight and cost. On the other hand, in the semiconductor laser array used as a luminous source, plural luminous sources are arranged at regular intervals in a monolithic manner. A light source unit is positioned and supported in an area orthogonal to an optical axis such that light beams from the luminous sources are arranged in the oscillating mirror and on an extended line of the twist beams on both ends of the oscillating mirror.
When a sub-scanning component of an arrangement pitch of the plural luminous sources is d′ and magnification of an imaging optical system in a sub-scanning direction is β, beam spot intervals p (mm) in the sub-scanning direction is expressed as: p=β·d′. On the other hand, when the number of luminous sources is n and movement speed of a surface to be scanned is v (mm/s), scanning line intervals p′ is expressed as: p′=v/n·f, where f (Hz) is a scanning frequency of the oscillating mirror. Originally, p=p′ is expected. However, the scanning frequency f is adjusted to a resonance frequency f0 of the oscillating mirror or set in a resonance band in the neighborhood of the resonance frequency f0, so that if dispersion of the resonance frequency f0 is large, intervals of adjacent scanning lines become uneven. In other words, the intervals of scanning lines set from the imaging magnification and the intervals of scanning lines across the oscillating mirror do not correspond to each other, so that unevenness of density is generated and this may be a factor in substantial derogation of image quality. Further, in a tandem multicolor image forming apparatus having plural image forming stations, when the beam spot intervals p are not accurately arranged in each station, color drift or color change may be generated.